Matrix-assisted laser desorption and ionization (MALDI) is a relatively new technique that allows very large molecules, such as DNA fragments and proteins, to be desorbed from a solid sample and ionized without significant decomposition. Coupled with mass spectrometry, this technique allows the molecular weights of biological polymers and other large molecules, including industrial polymers, to be precisely determined. One version of MALDI is described in a 1991 article in Rapid Communications in Mass Spectrometry, Vol. 5, Pages 198-202. A mass spectrometer suitable for obtaining highly reliable matrix-assisted laser desorption measurements is described in U.S. Pat. 5,045,694.
Most MALDI applications to date have employed time-of-flight mass spectrometers, although magnetic deflection, Fourier Transform ion cyclotron resonance, and quadrupole ion trap mass analyzers have also been used. A liquid solution of the sample to be analyzed is mixed with a solution containing an appropriate matrix, and a small aliquot of this mixtures is deposited on the source of the mass spectrometer (inside a vacuum system). A vacuum lock is generally utilized to avoid venting the vacuum system. Loading a sample typically requires from one to several minutes, and the attention of a skilled operator. A diligent operator should theoretically be able to load and run a sample every five or ten minutes using such a system, but it is difficult to maintain such a rate over an extended period. U.S. Pat. 5,288,644 discloses one technique for reducing the required time. A plurality of samples are loaded onto the solid surface of a disk, which is rotated by a stepper motor for positioning each sample respectively for striking by a laser beam.
Further improvements in the loading of samples for the laser desorption mass analysis are required for this analytical procedure to gain greater acceptance and significantly increase the use of this analytical tool. The disadvantages of the prior art overcome by the present invention, and an improved system is hereinafter disclosed for obtaining matrix-assisted mass spectrometer measurements. The loading of the samples is highly automated for achieving both high sample throughput and high reliability. The present invention has a wide range of application, and may be used with various analytical methods.